Understanding the Effect of a Paint Bake Cycle on the Microstructure-Mechanical Properties Relationship of a Resistance Spot Welded Advanced High Strength Steel

The effect of an automotive paint bake (PB) thermal cycle on the microstructural evolution and the mechanical properties of resistance spot welded advanced high strength steel is presented in this work. Mechanical behavior of the heat-treated welds reveals an increase in maximum cross-tension strength, displacement and subsequently energy absorption capability when 453K (180 degrees C)-20minutes a bake thermal cycle is applied after welding. The microstructures of resistance spot welds with and without a PB heat treatment were characterized using scanning and transmission electron microscopy (TEM). TEM analysis revealed that the weld nugget and HAZ of the resistance spot welds consist of a martensitic microstructure. The microstructural analysis of the post-weld heat-treated samples shows the presence of epsilon carbides in a martensitic matrix within the weld nugget and the HAZ. It is shown that the improved mechanical response of the paint-baked welds is associated with carbide precipitation during heat treatment.